Preventing system failure in solar PV systems

October 31st, 2014, Published in Articles: Vector

 

It is vital for solar photovoltaic (PV) systems to accommodate the temperature fluctuations and environmental conditions to which they are typically exposed as these conditions often cause ongoing system failure.

Fig. 1: Defining air mass.

Fig. 1: Defining air mass.

The difference between standard test conditions (STCs) and nominal operating cell temperature (NOCT) is of interest here as these values in PV modules determine the way in which equipment responds to power generated by the PV array under varying environmental conditions.

With 12, 24 and 48 V systems, the effect of temperature on voltage is not very prevalent or obvious, but most grid-tied inverters operate between 200 and  1000 V DC, where temperature fluctuations could have an impact on the voltage and power generated by the modules. The modules’ temperature co-efficient must be taken into account when these systems are designed and installed to ensure that the systems remain within inverter operating parameters.

The STC value defines the way in which PV modules are tested internationally. STC corresponds to 1000 W/m2 at 25°C cell temperature, with an air mass of 1,5 (AM 1,5), as defined in IEC 60904-3.

Air mass

Fig. 1 shows how air mass is defined. At the equator, the air mass would be 1 but AM 1,5 is used as a norm for PV test conditions, for purposes of comparison. The air mass is the path length which light takes through the atmosphere, normalised to the shortest possible path length (when the sun is directly overhead). The air mass quantifies the reduction in the power of light as it passes through the atmosphere and is absorbed by air, dust and other particles.

Nominal operating cell temperature

NOCT is a performance parameter and is defined for open-back, rack-mounted modules with a standard reference environment of a 45° tilt angle from the horizontal at total irradiance of 800 W/m2 and 20°C ambient temperature, and where a 1 m/s wind speed is available on a panel in an open circuit condition.

The NOCT value should be used by the system designer as a guide to determine the temperature at which a module will operate in the field and is a useful parameter when comparing the performance of modules and system designs.

Actual operating temperature could be affected by irradiance, ambient temperature, the type of mounting structure, prevailing wind speed, reflections and diffusion of light from the ground and nearby objects, among others.

The “primary method” to determine NOCT is an outdoor measurement method used by both IEC 61215 and IEC 61646, and is universally applicable to all PV modules. In the case of modules not designed for open rack mounting, the primary method may be used to determine the equilibrium mean solar cell junction temperature with the module mounted as recommended by the manufacturer.

The test setup requires data logging and selection for irradiance (pyronameter), ambient temperature (temperature sensors), cell temperature (thermocouples attached on the rear of the module corresponding to the two central cells), wind speed (speed sensor) and wind direction (direction sensor). All these quantities must be within certain boundaries to be acceptable for the calculation of NOCT. A minimum set of ten acceptable data points taken both before and after “solar noon” are used for the calculation of the final NOCT.

Calculation

To calculate the impact of temperature on PV power generation, you must know what the maximum power generation capacity is going to be under minimum temperature conditions, and what the minimum power generation capacity is going to be under maximum temperature conditions. The colder the ambient temperature, the more the power is generated and vice versa.

Some inverter manufacturers provide calculators to assist with the design of the PV system where minimum and maximum temperatures are added manually. These calculators generally distance the designer from the various parameters, although it is still very important to understand the parameters and to apply them accordingly, especially under fault-finding conditions.

The values for the formula should all be provided by the PV module manufacturer and are usually included in the solar panel’s data sheet. This data sheet will provide a voltage co-efficient value stipulated in mV or V. Some manufacturers may indicate the values as a percentage.

The highest possible voltage that can be achieved under most ideal conditions, i.e. high irradiation at low temperatures, is determined with the formula:

Vadj.max = ((Tlocalmin – Tstc) x Temp.Co-EffVoltageTemp) + Voc  [1]

An additional calculation may be needed where the co-efficient value is given as a percentage.

The resistance of copper conductors could also be affected, just as temperature has an impact on the efficiency of PV modules. It is vital to know where to obtain the required information to calculate voltage drop and to ensure properly-sized conductors for your electrical network, whether a solar PV system or an upgrade to an existing network.

South African conditions and calculations of voltage drop, as well as derating calculations, are addressed in SANS 10142-1:2006, page 310.

Contact Evert Swanepoel,  Copper Development Association,  Tel 011 824-3916, evert.swanepoel@copperalliance.org

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